Liquid crystal device operable in two modes and method of operating the same

ABSTRACT

A liquid crystal device is provided that comprises a layer of liquid crystal material ( 2 ) disposed between two cell walls ( 4, 6 ) in an arrangement such that the liquid crystal material ( 2 ) can adopt any one of two or more stable liquid crystal configurations that will persist in the absence of an applied electric field. The liquid crystal device is operable in two modes; a first mode in which application of an appropriate latching voltage pulse can select any one of the two or more stable liquid crystal configurations, and a second mode in which application of an electric field can switch the layer of liquid crystal material from a latched configuration to a switched configuration and in which the layer of liquid crystal material will return to said latched configuration when the applied electric field is removed.

This application is the US national phase of international applicationPCT/SE02/01199, filed in English on 18 Jun. 2002, which designated theUS. PCT/SE02/01199 claims priority to SE Application No. 0102151-8 filed18 Jun. 2001. The entire contents of these applications are incorporatedherein by reference.

TECHNICAL FIELD

This invention relates to a liquid crystal device, and in particular toa liquid crystal device which can operate in either of two distinctmodes.

BACKGROUND

Liquid crystal devices (LCDs) typically comprise a thin layer of liquidcrystal material contained between a pair of cell walls. The internalsurface of the cell walls are usually coated with a certain material, orare suitably adapted in some way, to impart a degree of surfacealignment to the liquid crystal. The bulk of the liquid crystal thenadopts a configuration that depends on the surface alignment propertiesof the cell walls and on various other factors, such as the type ofliquid crystal material and the thickness of the liquid crystal layer.Optically transparent electrode structures on one or both of the cellwalls allow an electric field to be applied to the liquid crystal layer.

A typical liquid crystal display device is designed such that two, ormore, liquid crystal configurations can be selected by the applicationof suitable electric fields. The different liquid crystal configurationsare designed to be optically distinguishable so that optical contrastcan be attained from the liquid crystal device. For example, a liquidcrystal device suitably arranged between a pair of polarisers may haveone configuration that will allow transmission of light through thesystem and a second configuration that will prevent it.

Monostable liquid crystal devices, in which the liquid crystal moleculescan only adopt one stable configuration, are known. Application of anelectric field can distort the configuration of the liquid crystalmolecules, but once the electric field is removed the liquid crystalwill relax back to its single stable configuration after somecharacteristic time (typically tens of milliseconds to a few seconds).

Twisted nematic (TN) and super-twisted nematic (STN) LCDs are examplesof monostable devices. The TN and STN devices may be switched to an “on”state by application of a suitable voltage, and will switch back to an“off” state when the applied voltage falls below a certain level. Itshould be noted that the terms “on” and “off” relate to application ofhigh (i.e. switching) voltage and low (i.e. non-switching) voltagerespectively not necessarily the observed optical transmission of adisplay. As these devices are monostable, loss of power leads to loss ofthe image.

Multi pixel TN and STN passive matrix displays can be constructed usingstripes of row and column electrodes on the upper and lower cellsurfaces which allows the device to be multiplexed. The drive voltagesapplied to the row and column electrodes are selected such that a numberof separate RMS voltage levels may be applied to each pixel of thedisplay. The optical transmission of a typical TN device variesnon-linearly with RMS voltage in a threshold transition manner asdetailed by Alt and Pleschko in IEEE Trans ED 21 1974 pages 146–155. Themaximum number of pixels that can be addressed using RMS methods isdictated by the optical transmission versus voltage characteristics ofthe TN or STN device, and in practice it has proven difficult to producepassive matrix STN displays with significantly more than about 500 linesof information due to cross-talk effects and manufacturing tolerances.It has also been demonstrated that other orthogonal functions, such asWalsh functions, can be used to passively address TN and STN displays.

Incorporating a driving element, such as thin film transistor (TFT) orother such non-linear elements (e.g. back-to-back diode, ferroelectriclayers etc), adjacent to each pixel in a TN LCD has been found tosignificantly increase the total number of pixels which can beaddressed; such displays are termed active matrix. In addition to theincreased number of pixels that can be incorporated in a TN display,active matrix TN devices have many other advantages compared withmultiplexed TN or STN displays; such as a relatively low operatingvoltage requirement, a wide temperature operating range and thecapability to provide greyscale. The speed with which TFTs can beswitched allows rapid sequential scanning through the row and columns ina display thereby permitting high speed, video rate, operation. As eachpixel is isolated using the TFT, the effect of cross-talk is effectivelyremoved in active matrix devices.

Many monostable devices permit grey-scale to be attained by controllingthe magnitude of voltage applied to the layer of liquid crystalmaterial. Selecting a TN liquid crystal configuration that has a shallowtransmission versus voltage characteristic, and applying a drive voltageto produce an intermediate transmission level, allows greyscale to beattained in active matrix TN devices. A storage capacitor may also beincluded with the pixel which allows the retention of charge, and hencemaintains the electric field across the liquid crystal for a limitedperiod of time. In a TN active matrix device the liquid crystal ismonostable, and as the electric field across the pixel decays it willreturn towards its relaxed configuration. Therefore, maintenance of animage on an active matrix TN display requires the regular updating ofeach pixel and hence a continual supply of power.

Active matrix TN devices are commonly used today in commercial displaysfor laptop computers, computer monitors, portable TV etc and devices areknown to those skilled in the art which can operate with multiple levelsof greyscale at video frame update rates. More detailed reviews ofactive matrix LCD technology have been prepared; see, for example, R. G.Stewart (1996) Active Matrix LCD, Society for Information Display,Seminar Lecture Notes, Volume 1, 13^(th) May 1996 San Diego ConventionCentre, Calif., M5-1-35.

The use of semiconductor circuits transferred to the device substratesusing printing techniques or fluidic self-assembly also allow theformation of discrete semiconductor circuits which are capable ofaddressing two or more display pixels. A review of such devices is givenin R. G. Stewart, (2000) Proceedings of the 20^(th) InternationalDisplays Research Conference, p415–418, held at Palm Beach Fla. USA,25–28 Sep. 2000.

U.S. Pat. No. 06,120,588 described how electro-phoretic inks can also beused in conjunction with TFTs. The use of the TFT active matrix removescross-talk effects from the electro-phoretic ink, which does not have asignificant threshold between its states. These devices are alsodescribed in K. Amundsen and P Drzaic (2000), Proceedings of the 20^(th)IDRC, 84–87.

Another known type of LCD device is a bistable device. In a bistableLCD, the liquid crystal material can adopt two different, and stable,configurations in the absence of an applied electric field. Researchinto bistable LCDs has been prompted mainly by their inherent ability tostore images and high multiplexibility. This negates the need fordevices that have expensive active matrix back-planes and permits lineat a time passive addressing.

Application of suitable electric fields to the bistable liquid crystallayer causes switching between the two stable configuration in which itcan exist; so called “latching”. Hereinafter “latching” shall be takento mean changing the liquid crystal from one stable configuration toanother stable configuration such that the state remains after theapplied voltage is removed, whilst “switching” shall mean any fieldinduced change to the configuration of the liquid crystal which includesmonostable switching effects.

Bistable liquid crystal devices are almost exclusively addressed usingpassive matrix techniques; the displays are constructed using stripes ofrow and column electrodes on the upper and lower cell surfaces whichallows the device to be multiplexed. The inherent ability to storeimages in the absence of power allows potentially complex images to bebuilt up a line at a time and also makes bistable devices attractive inapplications where low power consumption is required, for example inlaptop computers, PDAs and mobile telephone devices.

Examples of bistable liquid crystal displays include surface stabilisedferroelectric liquid crystal (SSFLC) devices as described by N A Clarkand S T Lagerwall, Appl. Phys. Lett., 36, 11, 899 (1980). Ferroelectricliquid crystal device are generally passively addressed but, because ofthe extremely rapid speed in which FLC materials can latch between thetwo stable configurations, they have also been combined with activematrix backplanes to produce devices having very fast frame times; see,for example, see J. Xue and M. A. Handschy, (2000) Proceedings of the20^(th) IDRC, p13–17.

It has also been demonstrated previously (see U.S. Pat. No. 5,604,616)that it is possible to operate a cholesteric display in a pseudobistable manner. The display can be electronically latched from a firststable state to a second stable state by application of a high voltage.Once the device is latched into the second state it is effectively“frozen” in that state and it is only possible to reliably reselect thefirst stable state by heating the device above the isotropic temperatureof the liquid crystal material. The first and second stable states aretwisted nematic configurations that possess a different degree ofoverall twist. Operating the device in what is termed the nematic modepermits RMS switching of device from either of the stable states. Thedevice of U.S. Pat. No. 5,604,616 thus provides none of the benefits oftruly bistable operation. For example, such a device does not allowimages that persist in the absence of applied electrical power to bewritten and subsequently rewritten many time using only electricaladdressing techniques.

It has also been shown in Patent Applications WO 91/11747 (“Bistableelectrochirally controlled liquid crystal optical device”) and WO92/00546 (“Nematic liquid crystal display with surface bistabilitycontrolled by a flexoelectric effect”) that a nematic liquid crystal canadopt, and can be switched between, two stable states via the use ofchiral ions or flexoelectric coupling.

WO 97/14990 teaches how a zenithally bistable device (ZBD) may beconstructed using a grating of a given design such that nematic liquidcrystal molecules can adopt two stable pretilt angles in the sameazimuthal plane. One of these states is a high pretilt state, whilst theother is a low pretilt state and a device is described which can adopt,and can be readily switched between, either of the two stable liquidcrystal configurations. WO99/34251 teaches another ZBD device having anegative dielectric anisotropy material in a twisted nematicconfiguration. Patent application GB0017953.1 describes a zenithallystable device exhibiting multi-stability rather than bistability.

The two stable liquid crystal configurations of ZBD persist afterdriving electrical signals have been removed, and (see Wood et. al. SIDDigest 2000) the device is highly resistant to mechanical shock,provides 10s of microsecond latching times at low driving voltages(<20V) and allows a high degree of multiplexibility. It has also beenshown, see Bryan-Brown et al, (1998) proceedings of Asia Display,p1051–1052, that grey-scale may be achieved using a chirped grating thatallows partial switching of a pixel.

Although bistable devices are ideal for low power and low costapplication, there is also a requirement in certain applications (suchas when displaying video images) to have a number of levels of greyin-between the dark and light states. Greyscale can be achieved in trulybistable devices using temporal and/or spatial dither where theperception of grey-levels is provided by switching each pixel “on” and“off” at a rate faster than the viewer can perceive or by dividing eachpixel into two or more weighted sub-pixel regions.

Employing spatial and/or temporal dither techniques in bistable displaydevices does however increase the complexity, and hence unit cost, ofdevices. For example spatial dither increases the number of row andcolumn drivers, requires thinner tracks thereby increasing trackresistance and resistive powers losses in the panel and also requiresmore accurate etching to ensure linearity of the greyscale response. Itis for these reasons, that passively addressed bistable devices known tothose skilled in the art are, for the present at least, somewhat limitedin there ability to produce high numbers of grey-levels and moving videoimages.

It is an object of this invention to mitigate some of the disadvantagesassociated with the liquid crystal devices described above.

SUMMARY

According to a first aspect of the invention, a liquid crystal devicecomprises a layer of liquid crystal material disposed between two cellwalls in an arrangement such that the liquid crystal material can adoptany one of two or more stable liquid crystal configurations that willpersist in the absence of an applied electric field, the liquid crystaldevice being operable in two modes; a first mode in which application ofan appropriate latching voltage pulse can select any one of the two ormore stable liquid crystal configurations, and a second mode in whichapplication of an electric field can switch the layer of liquid crystalmaterial from a latched configuration to a switched configuration and inwhich the layer of liquid crystal material will return to said latchedconfiguration when the applied electric field is removed.

A device according to the first aspect of this invention will thus, whenoperating in the first (multi-stable) mode, have the advantage of beingable to store images without the need for continual electricaladdressing of the device. Once an image is stored, it may be removed (oran alternative image written) by latching the device into the requiredstate. As described above, the ability to write (and re-write asrequired) an image to a device and for that image to then persist in theabsence of power can reduce the overall power consumption of the device.This is particularly advantageous when the device is incorporated inportable equipment, such as laptop computers, PDAs, digital cameras,electronic picture frames, mobile telephones etc.

A device according to this invention can also, when operating in thesecond mode, be switched from any one of the stable (i.e. latched)liquid crystal configurations into a “switched configuration”. Aswitched liquid crystal configuration will remain whilst power isapplied to the device, but the liquid crystal will decay back to thestable liquid crystal configuration when the applied field is removed.Second mode operation may provide a greater number of grey-levels or afaster switching rate than attainable from operation in the first (i.e.multistable) mode. In other words, switching between transient liquidcrystal states can be used to display images but such images requireperiodic updating.

A device according to this invention thus provides some of the benefitsnormally associated with only multi-stable or monostable devices. Forexample, low power operation can be achieved by operation in the firstmode whilst faster image updating and/or an increased number ofgrey-levels etc can be obtained by operation in the second mode.

Advantageously, the liquid crystal display device is latched in to aparticular stable configuration prior to operation in the second mode.Generally, the state that is selected is the state which is the mostdissimilar to that induced by the field in the second mode.

Preferably, the arrangement is such that the liquid crystal material canadopt any one of three or more stable liquid crystal configurations.Latching voltage pulses can then be provided to switch between any ofthe three or more stable configurations.

Conveniently, the arrangement is such that the liquid crystal materialcan adopt any one of two stable liquid crystal configurations. In otherwords, the device is bistable and either of the two stable liquidcrystal configurations can be selected by application of suitablelatching voltage pulses.

Advantageously, a surface alignment grating is provided on the internalsurface of at least one cell wall that is adapted to provide two stableliquid crystal configurations. Surface alignment gratings that providezenithal bistability are described in the prior art; see for example WO97/14990.

Conveniently, the liquid crystal material comprises a nematic liquidcrystal material.

Preferably, the liquid crystal device may comprise a surface alignmentgrating provided on the internal surface of the first cell wall and aplanar surface treatment provided on the internal surface of the secondcell wall, the arrangement being such that one of the stable liquidcrystal configurations is a twisted nematic (TN) configuration.

In other words, one of the two stable alignment configurations inducedby the surface alignment grating provides substantially planar alignmentof the liquid crystal at one cell wall, and a homogenous surfacetreatment on the other cell wall induces substantially planar alignmentof the liquid crystal at that wall. The in-plane alignment direction ofthe substantially planar alignment induced by the surface alignmentgrating at one cell wall is adapted to be substantially different to thein-plane alignment direction induced by the homogenous surface treatmentat the other cell wall such that one of the stable liquid crystalconfigurations adopted is a twisted nematic configuration.

Herein the term twisted nematic (or TN) configuration means aconfiguration in which the liquid crystal twists from a orientation atone cell wall to a second orientation at the other cell wall. The termincludes so-called super twist (STN) structures and the like.

Conveniently, the twist of the liquid crystal material from the firstcell wall to the second cell wall in the twisted nematic configurationis greater than 45° or greater than 90° or greater than 180° or greaterthan 270°.

Advantageously, the device is latched into the twisted nematicconfiguration prior to operation in the second mode.

In this way, a zenithally bistable device is provided with one stableliquid crystal configuration being a twisted nematic (TN) configuration.This TN configuration will switch in response to the RMS voltage in asimilar manner to the monostable TN structures described in the priorart. In other words, a device is provided which can be latched intoeither of two bistable states by an electrical pulse and which can alsoswitch in response to applied voltages in the same way as as amonostable TN device. Advantages of TN devices are thus combined withadvantages of electrically latched bistable operation.

Advantageously, the liquid crystal device comprises a surface alignmentgrating provided on the internal surface of the first cell wall and ahomeotropic surface treatment provided on the internal surface of thesecond cell wall, the arrangement being such that one of the stableliquid crystal configurations is a vertically aligned nematicconfiguration.

In other words, the surface alignment grating on the internal surface ofone cell wall is adapted such that it induces substantially homeotropicalignment, and a surface treatment on the other cell wall inducessubstantially homeotropic alignment of the liquid crystal, such that oneof the stable liquid crystal configurations adopted is a verticallyaligned nematic (VAN) configuration.

A person skilled in the art would appreciate that the term verticallyaligned nematic structure means any liquid crystal configuration inwhich the liquid crystal director throughout the device is oriented in asubstantially perpendicular direction to the cell walls of the device.In other words, the nematic liquid crystal is vertically aligned withreference to the horizontal cell walls.

Conveniently, the device is latched into the vertically aligned nematicconfiguration prior to operation in the second mode.

Preferably, the liquid crystal material comprises a nematic (or longpitch cholesteric) material. Conveniently, the liquid crystal materialmay further comprise cholesteric liquid crystal material.Advantageously, the liquid crystal material comprises less than 0.02% byweight of cholesteric liquid crystal material.

Advantageously, the ratio of liquid crystal layer thickness to the pitchof liquid crystal material is greater than 0.25 and/or less than 1.25.

Inclusion of a cholesteric dopant will enable a twisted nematicstructure having a twist of greater than 90° to be readily achieved. ATN structure having a twist greater than 90° will give an STN like steepelectro-optic switching threshold. This steep threshold reducescross-talk effects and enables a larger number of lines to be passivelyaddressed in the continuous mode.

Preferably, the liquid crystal material has a negative dielectricanisotropy or alternatively a positive dielectric anisotropy.

Preferably, the cell walls comprise electrodes for applying an electricfield to the liquid crystal, the electrodes being arranged to form amatrix of addressable pixels.

Conveniently, the device comprises row electrodes formed on the firstcell wall and column electrodes formed on the second cell wall, thefirst and second cell walls being arranged so as to define an array ofpassively addressable pixels. The display may then be multiplexed byapplication of appropriate row and column voltage waveforms.

Advantageously, the device comprises a means of independently applyingan electric field to each individual pixel. In other words, an activebackplane may be provided.

Conveniently, the means of independently applying an electric field toeach individual pixel comprises a thin film transistor element, whichmay advantageously have a storage capacitor associated therewith. Thestorage capacitor allows charge to be stored so that an electric fieldis held across the liquid crystal for a period of time; for the reasonsdescribed above in relation to active matrix monostable devices suchcharge storage is advantageous for device operability in the secondmode.

In a further preferred embodiment, the device includes at least onediscrete semiconductor circuit capable of addressing two or more displaypixels. Such semiconductor circuits are described in R. G. Stewart,(2000) Proceedings of the 20^(th) International Displays ResearchConference, p415–418, held at Palm Beach Fla. USA, 25–28 Sep. 2000.

Preferably, the device is adapted such that the first mode of operationand the second mode of operation are capable of simultaneous use ondifferent pixels or groups of pixels.

Advantageously, pixels that are to be operated in the second mode may belatched into a predetermined stable liquid crystal configuration (i.e.blanked) prior to operation in the second mode.

A given portion of a display device may thus be operated in the firstmode whilst a another portion is operated in the second mode. In thecase of a display, this allows certain areas of the device to takeadvantage of the higher switching speed or increased number ofgrey-levels of second mode operation whilst conserving power to someextent by operating the remainder of the display in the first mode. Forexample, a laptop computer display may be required to show a moving orhigh resolution image only in a defined area, or window, of the display.

Conveniently, the driving voltages applied to the device aresubstantially d.c. balanced over time. This prevents degradation of theliquid crystal material.

Preferably, the second mode is adapted to display images at video rate(i.e. 50 or more frames per second) and/or with at least 64 or 256grey-levels. The second mode may also be operated at a lower rate (e.g.10 frames a second) to enable black and white animation to be displayed.

In a further preferred embodiment, the device comprises at least onepolariser to distinguish between different liquid crystalconfigurations. A person skilled in the art would recognise the variousway or ways in which one or two polarisers could be arranged, possiblyin conjunction with additional optical elements (e.g. retardation filmsetc) such that optical contrast could be obtained for the various liquidcrystal configurations. It would also be recognised that a dye may alsobe included in the liquid crystal material in addition to, or insteadof, providing a polariser.

Advantageously, the device further comprises a reflective means and thedevice is configured to operate in reflective mode. The reflective meansmay comprise a specularly reflective layer, or a reflective layer on onecell wall combined with a diffuser on the other cell wall.

The liquid crystal display device may additionally comprise colourfilter elements. Alternatively, colour reflectors or coloured absorbersor coloured polarised could be employed as required. A colour displaydevice may thus be obtained.

Preferably, the device further comprises means for selecting first modeor second mode operation.

For example, means may be provided that differentiates betweeninformation relating to static pictures and video signals. For exampleareas of text on a page of a computer screen may be addressed in storagemode, whilst moving images are addressed in continuous mode.Alternatively, the means may select the mode of operation by taking intoaccount the format of data to be displayed. For example, when the devicedisplays a clock it may always display seconds in the continuous mode,but the minutes and hours and other information (e.g. date, icon etc) inthe storage mode.

It is also possible to operate the device such that one line ofinformation (e.g. the line of text currently being edited) is updated inthe second mode whilst the remainder of the display (i.e. text that isnot in the process of being edited) is displayed using the first mode. Aperson skilled in the art would recognise how signals to select firstmode or second mode operation as appropriate could be derived from thegraphics hardware and/or software that is used to construct the imagedata for display. This technique is particularly advantageous fordevices in which the contrast decreases substantially during first mode(i.e. latching) operation.

According to a second aspect of the invention, an information displayapparatus comprises a microprocessor unit adapted to electronicallyaddress a liquid crystal device according to the first aspect of theinvention. The information display apparatus may be a laptop computer,PDA, digital camera, electronic picture frame, mobile telephone etc.

According to a third aspect of the invention, a method of operating aliquid crystal device comprising the steps of; taking a liquid crystaldevice that is arranged such that the liquid crystal material can adoptany one of two or more stable liquid crystal configurations that willpersist in the absence of an applied electric field, and determiningwhether to operate the device in a first mode or a second mode, wherebyin the first mode of operation any one of the two or more stable liquidcrystal configurations can be selected by application of an appropriatelatching voltage, and in the second mode of operation the application ofan electric field can switch the layer of liquid crystal material from alatched configuration to a switched configuration and in which the layerof liquid crystal material will return to said latched configurationwhen the applied electric field is removed.

Conveniently, the liquid crystal device comprises a plurality of pixelsand in which only some of the pixels are operated in the second mode.

Advantageously, the liquid crystal material is latched in to aparticular stable configuration prior to operation in the second mode.

Preferably, the method is performed on a suitable configured zenithalbistable device.

The present invention thus provides a liquid crystal display devicewhich is operable in two modes, wherein the first mode is a muti-stablemode such that images persist in the absence of an electric field, andthe second mode offers a greater number of grey levels and/or fasterswitching speeds and/or addressing without unwanted contrast variationsand/or the ability to display animated images than the first mode whenappropriate electric fields are applied to the device.

In other words, a liquid crystal device (LCD) comprises; a layer ofliquid crystal material contained between two cell walls and a means ofapplying an electric field thereto, means of optically distinguishingdifferent configurations of the liquid crystal layer wherein the LCD isadapted such that under the influence of an electric field the opticaltransmission properties of the device are altered in a controlledmanner, and where differing electric fields produce a plurality oftransmission levels, and where the layer of liquid crystal material isalso bistable such that under no applied field the liquid crystalmaterial adopts either of two configurations and where the applicationof a voltage pulse of suitable polarity, magnitude and direction causesthe liquid crystal material to latch into either one of its two bistablestates.

Furthermore, this invention also provides a method of operating a liquidcrystal device in two modes, said method comprising the step ofoperating the device in a latching mode such that two or more stableliquid crystal configurations may be selected, said stable liquidcrystal configurations persisting in the absence of an applied voltage;said method additionally or alternatively comprising the step ofoperating the device in a second mode such that the application of anelectric field provides a plurality of transient liquid crystalconfigurations, whereby the step of operating the LCD in latching modeis performed to switch the LCD into a stable configuration such that theselected stable configuration will persist without the requirement tocontinually expend power.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by reference to the accompanyingdrawings, in which;

FIG. 1 shows a schematic illustration of a prior art ZBD,

FIG. 2 shows the switching characteristics of a prior art ZBD,

FIG. 3 is the transmission vs voltage response of a device according tothis invention,

FIG. 4 is a schematic illustration of a prior art TN active matrixdevice,

FIG. 5 shows an active matrix device according to the present invention,

FIG. 6 illustrates a passive addressing regime,

FIG. 7 shows a comparison of change in transmission of a ZBD test cellwith applied RMS voltage and latching voltage,

FIG. 8 shows transmission versus RMS voltage for the test cell describedwith reference to FIG. 7,

FIG. 9 shows a plan view of a matrix multiplexed liquid crystal displayaccording to the present invention, and

FIG. 10 shows the cross section of the display of FIG. 9.

DETAILED DESCRIPTION

Referring to FIG. 1, a zenithal bistable device of the type describedWO99/34251 is shown. A layer of nematic liquid crystal material 2 havinga negative dielectric anisotropy is sandwiched between a first glasswall 4 and a second glass wall 6. The first glass wall 4 is coated witha bistable surface alignment grating 8 which is coated with ahomeotropic surfactant; as described in WO97/14990 and WO99/34251 theliquid crystal in the vicinity of the granting surface may then adopteither a non-defect or defect alignment configuration. The second glasscell wall 6 is treated, for example by coating with a polymer that issubsequently rubbed, to induce planer surface alignment of the nematicliquid crystal material 2 at the second glass wall 6.

The non-defect and defect structures that are induced by the surfacealignment grating are shown in FIGS. 1 a and 1 b respectively.

In the non-defect structure of FIG. 1 a the nematic liquid crystal will,at the interface between the surface alignment grating 8 and the liquidcrystal 2, orient so as to be substantially perpendicular to the localsurface of the grating. Within a short distance of the grating-liquidcrystal interface, as compared with the overall thickness of the liquidcrystal cell, the liquid crystal will adopt a substantially homeotropicalignment configuration. Homogenous alignment of the liquid crystal isinduced at the second glass wall 6, and a splayed liquid crystalconfiguration is thus adopted.

In the defect structure of FIG. 1 b, so-called defects or disclinationswill form in the vicinity of concave and convex defect sites. The resultof the formation of defect pairs is that within a short distance of thegrating-liquid crystal interface, as compared with the overall thicknessof the liquid crystal cell, the nematic liquid crystal will adopt aconfiguration with a pretilt substantially less the homeotropicconfiguration. In this configuration, a 60° twisted structure is formed,which has low (typically of the order of 20°) out of plane tilt.

WO99/34251 describes how such a structure, when formed from a nematicmaterial having a negative dielectric anisotropy, reduces unwanted RMSvoltage effects and enhances bistable performance.

Referring to FIG. 2, the switching characteristics of a device as shownin FIG. 1 are presented in the form of a graph depicting the voltage andwidth of switching pulse necessary to switch the device between thedefect and non-defect states. This is described in more detail elsewhere(see e.g. WO97/14990, WO99/34251).

Placing a device of the type shown in FIG. 1 between a pair of crossedpolarisers, for example, allows optical contrast to be achieved betweenthe splayed non-defect state of FIG. 1 a and the twisted defect state ofFIG. 1 b. It would be immediately apparent to a person skilled in theart how to optimise the cell thickness, for a liquid crystal material ofa given birefringence, to maximise the contract obtained. A personskilled in the art would also recognise that a plurality of alternativeoptical systems could be used to exploit the difference in opticalproperties of the defect and non-defect states.

As described above, it is difficult to achieve the 256 grey-levelsneeded for full colour images with bistable devices and one means ofachieving this is to use spatial dither. For example, digital weightingof the rows in the ratio 1:2 and columns in the ratio 1:4 allows 8levels to be achieved, even though each pixel has only 2 inherentlevels. If an intermediate (or analogue) level is possible thenweighting the rows and columns in ratios of 1:3 and 1:9 respectivelyallows a maximum of 81 grey levels to be achieved. This is stillinsufficient for full colour applications. It becomes possible todisplay 256 static grey levels if a 4^(th) level is possible byweighting the rows in the ratio 1:4 and the columns in the ratio 1:16.

However, there are a number of problems with using spatial dither inthis fashion. It doubles the number of row drivers and the number ofcolumn drivers, representing a major cost to the overall module cost.Losses along the thin tracks also become substantial. For example, witha 250 μm pixel pitch and a 10 μm minimum etch, the 4 level ZBD requiresa minimum sub-pixel width of 13.5 μm. Although possible to manufacture,the track resistance for a large panel can be very high indeed and thereare significant losses of the signal down the line. Also, etching errors(particularly on the least significant bit) can lead to non-linear orloss of grey levels.

Another important requirement for display devices is the need for videooperation, usually with a frame rate of about 50 Hz. Video operation iseasy with TFT driven LCD due to the high speed of the transistor switch,provided that the liquid crystal material and cell gap are chosen togive suitably fast optical response. With a bistable display device,such as ZBD, the frame rate is dictated by two factors: theline-address-time multiplied by the number of lines, and the slower ofthe optical response times. For very complex displays with a largenumber of lines, it is therefore difficult to achieve video rate (i.e.50 Hz) operation. This situation is worse using spatial dither, sincethere may be at least twice as many addressable rows in the display.

The present invention provides a device that, in common with prior artZBD devices, can latch into either one of the two stable ZBD states; itcan operate in bistable, or image storage, mode. In addition, a deviceof the present invention is also designed to exhibit a RMS voltageresponse at sub-latching voltages; i.e. it also operates in asub-latching, or continuous, mode. In other words, the device can beoperated both as a bistable device and in a RMS mode. This dual mode ofdevice operation runs contrary to the present thinking in the art thatthe mode of operation, and hence electrical addressing, of monostableand bistable devices are quite distinct.

Referring to FIG. 3 a, the voltage versus transmission responsecharacteristic of a device according to the present invention is shown.Curve 14 represents the RMS response of the liquid crystal in the bulkof the cell, and is analogous to a standard TN response. Curve 16represents the change in transmission as the device latches from thedefect to the non-defect state, and curve 18 represents the change intransmission as the device latches from the non-defect to the defectstate. As described in more detail below, devices according to thepresent invention are designed such that bistable switching (i.e.latching) and RMS switching (i.e. sub-latching voltage induceddistortion of the liquid crystal) are separated so that both can beexploited discriminately in a single device.

The type of behaviour shown in FIG. 3 a is achieved using a twistednematic ZBD geometry of the type described with reference to FIG. 1above. However, unlike the ZBD device described with reference to FIG. 1the dielectric anisotropy of the liquid crystal material in this deviceis preferably (although not necessarily) positive. The mono-stablesurface has low pre-tilt, planar homogeneous alignment arrangedperpendicular to the director in the defect state of the zenithalbistable surface. The device in this instance acts in normally whitemode when it is placed between crossed polarisers. Once blanked into thedefect (low tilt) state, the whole pixel appears transmissive due to thetwisted nematic structure in the bulk of the liquid crystal.

Application of a voltage higher than the TN transition voltage V1(either positive or negative since it is an RMS response) causes thedirector to reorient in the bulk of the cell with negligible change ateither surface. A typical TN transmission characteristic saturates intothe dark state around ±V2. Typical values for V1 and V2 are 1.5V and4.5V, respectively.

Although the monopolar pulse applied to the liquid crystal in RMS modeis of the correct sign to cause latching into the non-defect state, thedevice is designed such that V2 is not sufficient to cause any latchingof the surface; i.e. to produce latching from the defect to non-defectconfiguration requires a voltage pulse greater than τV3 to be applied.It should be noted that latching occurs when a pulse of sufficientvoltage and duration is applied to the device (i.e. in this case a pulsegreater than τV3), whereas the RMS response depends purely on the RMSvoltage level and is independent of the polarity.

To ensure (approximate) d.c. balancing the device may, for example, belatched using an appropriate negative pulse at the start of each frame.This also ensures that the correct stable state is rewritten at thestart of each frame to help reduce errors.

Referring to FIG. 3 b, a voltage versus transmission response is givenfor a device in which the bistable grating is designed to provide anasymmetric latching response; teaching on how to obtain such a gratingdesign is found in the prior art, for example see WO97/14990. Curve 14represents the RMS response of the liquid crystal in the bulk of thecell, and is analogous to a standard TN response. Curve 17 representsthe change in transmission as the device latches from the defect to thenon-defect state, and curve 19 represents the change in transmission asthe device latches from the non-defect to the defect state.

In this device, latching from the non-defect state to the defect state(i.e. applying a pulse of voltage of less than −τV3) can be obtainedwith a similar magnitude of voltage pulse that is required to exploitthe RMS response. In this manner, the device can be addressed whilstmaintaining substantial d.c. balance.

A device of the present invention can thus operate in either of twomodes: continual update mode or image storage mode. The presentinvention can thus provide a display that is capable of full colour,video operation and which has the ultra-low power consumption of adevice that has true image storage. A person skilled in the art wouldalso immediately recognise that the use of a device of the presentinvention could prove advantageous in any application where themodulation of radiation is required; e.g. as a spatial light modulator,optical shutter etc.

The display is designed such that these modes have separatevoltage/charge operating ranges. This is done through choice of LCmaterial (e.g. elastic properties, dielectric anisotropy), gratingdesign (e.g. grating pitch, depth, shape), surface anchoring propertiesand cell gap so that latching into both bistable states occurs forvoltages greater than the saturation voltage for changes intransmission. The control of the relative voltages require for continualupdate or storage mode is possible because latching of the ZBD displayis a field effect (being associated with the flexo-electricpolarization) whereas the dynamic RMS response of the transmission is avoltage effect, and thus independent of cell gap.

Once the display is switched into storage mode, the last frame ofinformation is read from the frame store, but encoded into a suitablelevel of complexity (eg B/W or a limited number of grey-levels) thedisplay is again blanked and selectively written with voltages whichexceed +τV4 for pixels to be latched into the other stable state. Thisimage will then be retained until the next update of the frame, eventhough the device may be disconnected from the power. This may be doneautomatically on power off to a display.

To implement this invention a TFT active matrix can be used and FIG. 4shows a typical prior art TFT TN device 20. Four TFT driven pixels areillustrated 22, 24, 26, 28 together with row electrodes 30, 32 andcolumn electrodes 34, 36. The device shown operates in normally whitemode (i.e. the pixel remains white where the column voltage is zero).Grey levels may be achieved by amplitude modulation of the signalsapplied to the column electrodes.

The transistor that is associated with each pixel in an active matrixdisplay generally has two electrical input connections; a gateconnection 38 and a drive connection 40. Application of a suitablevoltage to the gate connection 38 applies the voltage present on thedrive connection 40 to the transparent pixel electrode 22. It is alsopossible to form a grating alignment structure (i.e. a structure thatcan impart a degree of alignment to a liquid crystal material) in one ormore of the layers used to form the TFT.

Active matrix displays are typically arranged with the gate connectionsof each pixel transistor in a row connected to a common row electrode,and the drive connections of each transistor in a column connected to acommon column electrode. Application of a gate voltage to a single rowin combination with the application of a driving voltage to a singlecolumn thus allows a driving voltage to be applied across the liquidcrystal layer of an individual pixel. Suitable drive and gate voltagepulses to address the 4 pixel device of FIG. 4 are also shown.

Referring to FIG. 5, TFT addressing schemes suitable for driving adevice according to the present invention are given.

FIG. 5 a) shows the active matrix addressing schemes required foroperation of a dual mode device 50 according to this invention incontinual update mode. During any period in which a voltage pulse isapplied to the gate connection 38, the voltage applied to the driveconnection 40 is applied to the electrode of the pixel 22.

Using the driving scheme shown in FIG. 5 a, it can be seen that thepixel 22 is initially latched into one bistable state (i.e. blanked)before a voltage is applied which switches the liquid crystal in to thedesired configuration. Ideally the pixel is blanked into the stablestate which will provide the best change in optical contrast onsubsequent application of a RMS voltage. In this mode the deviceoperates in a manner analogous to a TN device; any voltage (below thelatching voltage τV3) will cause a distortion of the liquid crystalconfiguration. As this voltage decays, the liquid crystal will relaxtoward the initial (i.e. blanked) stable configuration.

The pixels of a device according to this invention can be written eachrow sequentially as in the prior art, ensuring that the column voltagenever supercedes τV3 and thereby no latching into the other domainoccurs. Note, unlike conventional TFT drive schemes, the polarity of thecolumn voltages is kept the same in all frames (rather than alternatingpolarity from frame to frame to give overall DC balancing). A personskilled in the art would recognise the various mechanisms through whichd.c. balance could be obtained over time; for example a variable voltageblanking pulse or the use of a d.c. balancing pre-pulse.

FIG. 5 b) shows the active matrix addressing schemes required foroperation of a dual mode device 50 according to this invention in imagestorage mode. Using the driving scheme shown in FIG. 5 b, it can be seenthat the pixel 22 is initially written into one bistable state (i.e.blanked) before a voltage is applied which latches the liquid crystal into the other state. In storage mode, blanking may be done as forcontinuous mode, but the subsequent drive voltage required for pixels tobe latched into the other state must surpass τV3 so that latchingoccurs. In the example shown in FIG. 5 b, the column voltage in thesepixels is higher than τV4 so that complete latching of the pixel isachieved.

For a device operating in storage mode, it may also be possible to givestored analogue greyscale. For example, a chirped grating structurecould be used as described in Bryan-Brown et al, (1998) proceedings ofAsia Display, p1051–1052. Sub-pixellation may also be provided toprovide spatial dither. In fact, a person skilled in the art wouldrecognise that techniques used to obtain analogue greyscale in abistable device (e.g. exploitation of the partial switching region)could also be used to obtain grey-scale in a device according to thisinvention that is operating in storage mode. Similarly, it would berecognised by a person skilled in the art that temporal dither could beapplied to provide grey-scale during continuous mode operation.

When operating in continual update mode, it is likely that a displayaccording to this invention will be used in front-lit reflective mode.For storage mode, it is likely that the display will be required tooperate in a reflective mode without any illumination to furtherconserve power. Therefore, the liquid crystal layer would ideally betailored to operate in a reflective mode. However, and depending on theexact operating environment of the device, the device could also beoptimised to work in transmission mode. Techniques of optimising adevice for such purposes would be well known to a person skilled in theart.

In one embodiment of the invention, a single internal reflectorpolariser is provided, having an internal light controlling reflectorwith TFT deposited underneath the ZBD surface and a colour filter plateat the front surface. Alternatively, a specular reflective plate on therear surface can be used in conjunction with a diffusive sheet at thefront of the panel. Although the ZBD grating can be located on eitherthe active or common plates, for ease of manufacture it may bepreferable for the grating to be fabricated on the common plate.Alternatively, the required grating profile features may be fabricatedsimultaneously with one of the masked steps used in processing the TFT.

Referring to FIG. 6, a technique for passively addressing a ZBD testcell according to the present invention is shown. The test cell has a1.5 μm cell gap, and is filled with the commercially available nematicliquid crystal material MLC 6204. Curve 60 shows the voltage requiredfor a given duration of bi-polar pulse to fully latch the device fromthe defect (i.e. TN) state to the non-defect state, whilst curve 62shows the onset of latching. A partial latching region 64 is thusdefined.

As described above, passive matrix displays are typically constructedusing stripes of row and column electrodes located on the upper andlower cell surfaces respectively. This provides a matrix of addressablepixel elements. The resultant voltage applied across the liquid crystalof a single pixel is then simply the difference of the voltages appliedto the relevant column and row electrodes. Typically strobe waveformsare sequentially applied to rows, whilst data waveform (e.g. a select ornon-select waveform) are sequentially applied to the columns. In thismanner, each pixel in the display can be addressed (or multiplexed) inturn. Multiplexing techniques of this type are well known to thoseskilled in the art.

A strobe waveform can thus be combined with a variety of data waveformsin order to define the response of the particular pixel. Taking a 400 μsbi-polar strobe pulse of 11V amplitude, and combining it with −1V and+1V data pulses of a similar duration will produce a resultant voltagepulse at the relevant pixel of 12V (pt B of FIG. 6) and 10V (pt A ofFIG. 6) respectively. For an identical strobe pulse, application of−2.7V and +2.7V data pulses will produce resultant voltage pulses at therelevant pixel of 13.7V (pt C of FIG. 6) and 8.3V (pt D of FIG. 6)respectively. It can thus be seen that data voltages of ±1V can be usedto provide continuous mode operation (i.e. pt A and B) whilst datavoltages of ±2.7V provide resultant pulses that provide select andnon-select latching.

Referring to FIG. 7, the change in transmission of a ZBD test cellformed using a bistable grating with a pitch of 0.9 μm is shown. Theline 80 shows the change in transmission of the cell (in arbitraryunits) with the application of various magnitude latching voltage pulsesof 500 μs duration. The line 82 shows the change in transmission withapplied RMS switching voltage. In both cases, the TN state was selectedprior to taking the measurements.

Referring to FIG. 8, the transmission (arbitrary units) versus RMSvoltage data for the test cell described with reference to FIG. 7 isshown in an expanded view. It can thus be seen that 90% and 10%transmission will occur for this test cell with the application of RMSvoltages of 0.7V (termed V_(off)) and 1.6V (termed V_(on)) respectively.

Taking the electro-optic response for continuous RMS switching shownwith reference to FIGS. 7 and 8, the maximum number of lines that can bemultiplexed (n_(max)) can be estimated using the expression:$\begin{matrix}{n_{\max} = \frac{\left( {V_{ON}^{2} + V_{OFF}^{2}} \right)^{2}}{\left( {V_{ON}^{2} - V_{OFF}^{2}} \right)^{2}}} & (1)\end{matrix}$with the data V_(d) and strobe V_(S) signals approximately related by:$\begin{matrix}{\frac{V_{d}}{V_{S}} = \frac{1}{\sqrt{n_{\max}}}} & (2)\end{matrix}$

For the results shown in FIG. 8, equations (1) and (2) estimaten_(max)=2 and V_(d)=0.71V_(S). For the continuous mode to operatewithout unwanted latching, the following condition will hold:τ_(C)(V _(S) +V _(d))<τ_(S) V _(th)  (3)where V_(th) is the threshold for the latching amplitude for a pulse ofduration τ_(S), and τ_(C) is the duration of the continuous mode pulse.

Taking the results of FIG. 7 and choosing τ_(C)=τ_(S)=500 μs, then tworows may be addressed continuously (e.g. to show animation) withoutcausing unwanted latching provided that V_(S)<0.586V_(th). In this case(V_(th)=22V), V_(S)=12.5 V, V_(d)=8.8V and the data required to latchinto storage mode V_(D) must be greater than 9.5V. It would berecognised that different voltages may be chosen by appropriateadjustment of τ_(S), V_(th) and V_(S).

Referring to FIGS. 9 and 10, a passively addressed display according tothe present invention is shown.

The liquid crystal cell 101 is formed by a layer 102 of liquid crystalmaterial contained between walls 103, 104 which may be any suitablematerial for instance glass and/or plastic. Silicon or metal could alsobe used if the device were to be operated in reflective mode. Spacers105 distributed appropriately throughout the cell maintain the walls therequired distance apart. Strip like row electrodes 106, which may be,for example SnO₂, indium tin oxide (ITO) or Aluminium, are formed on onewall 103 and similar column electrodes 107 are formed on the other wall104. With m-row and n-column electrodes this forms an m×n matrix ofaddressable elements or pixels formed by the intersection of a row and acolumn electrode.

A row driver 108 supplies voltage to each row electrode 106. Similarly acolumn driver 109 supplies voltage to each column electrode 107. Controlof applied voltages is carried out by control logic 110 connected tovoltage source 111 and clock 112.

Either side of the cell are polarisers 113, 113′ which may, depending onthe particular liquid crystal arrangement, be placed with theirpolarisation axis substantially crossed with respect to one another andat an angle of substantially 45° to the alignment direction R, if any,on the adjacent wall 103, 104. Additionally one or more opticalcompensation layers 117 of, for example, stretched polymer may be addedadjacent the liquid crystal layer 102 between cell wall and polariser.Of course, the skilled person will be aware of other embodiments thatcould be implemented using one polariser or no polarisers at all.

A partly reflecting mirror 116 may be arranged behind the cell 101together with a light source 115. These allow the display to be seen inreflection and lit from behind in dull ambient lighting. For atransmissive device the mirror 116 may be omitted. Alternatively aninternal reflecting surface may be used such as an internal Aluminiumelectrode.

Prior to assembly at least one of the cell walls 103, 104 are providedwith a surface alignment grating to provide a bistable pretilt. Theother surface may be provided with either a planar, tilted orhomeotropic monostable surface or another bistable surface. The surfacealignment grating structures providing bistable pretilt may bemanufactured using a variety of techniques as described above.

Although the above embodiments disclose the use of a 90° twisted ZBDcell operating using a positive material, a person skilled in the artwould immediately recognise that a plurality of alternative ZBDconfigurations could be used. For example, a cholesteric dopant could beadded to the nematic liquid crystal material to induce a twist ofgreater than 90°. This gives an “STN-like” steep electro-optic switchingthreshold and enables a larger number of lines to be passively addressedin the continuous mode. For typical liquid crystal mixtures this wouldrequire less than 0.02% of cholesteric additive in the nematic host(usually giving d/P<1.25 where d=cell spacing, and P is the naturalpitch of the nematic/cholesteric mixture).

A negative material could be used in conjunction with a homeotropicmonostable surface; the blanking pulse should then chosen to latch intothe non-defect state and the continuous mode will operate as avertically aligned nematic (VAN) cell.

A planar (or tilted planar) monostable surface (e.g. a rubbed polymer)could also be used opposite a bistable surface alignment grating toprovide a bistable device having a hybrid (non-defect) state and anuntwisted planar (defect) state. A liquid crystal material having apositive or negative dielectric anisotropy could be used in thisconfiguration. The cell spacing and liquid crystal birefringence couldbe tailored such that the hybrid state acts as a quarter wave-plate(i.e. provide optical retardation of λ/4), whilst the planar state actsas a half wave-plate (i.e. provides optical retardation of λ/4). Thedisplay could then be operated in transmission mode by placing itbetween crossed polarisers aligned at 45° to the alignment direction.Alternatively, a mirror and single polariser could be provided forreflective mode operation.

A skilled person would recognise that alternative ZBD devices could beprovided, and that this invention is not only applicable to such ZBDdevices. Devices having a plurality of stable configurations, and whichare configured so that switching can also occur, can be operated inaccordance with this invention.

1. A liquid crystal device comprising, a layer of liquid crystalmaterial disposed between two cell walls in an arrangement such that theliquid crystal material can adopt any one of two or more stable liquidcrystal configurations that will persist in the absence of an appliedelectric field, the liquid crystal device being operable in two modes; afirst mode in which application of an appropriate latching voltage pulsecan select any one of the two or more stable liquid crystalconfigurations, and a second mode in which application of an electricfield can switch the layer of liquid crystal material from a latchedconfiguration to a switched configuration and in which the layer ofliquid crystal material will return to said latched configuration whenthe applied electric field is removed.
 2. A liquid crystal deviceaccording to claim 1 wherein the device is latched in to a particularstable configuration prior to operation in the second mode.
 3. A liquidcrystal device as claimed in claim 1 wherein the arrangement is suchthat the liquid crystal material can adopt any one of three or morestable liquid crystal configurations.
 4. A liquid crystal device asclaimed in claim 1 wherein the arrangement is such that the liquidcrystal material can adopt any one of two stable liquid crystalconfigurations.
 5. A liquid crystal device as claimed in claim 4,wherein a surface alignment grating is provided on the internal surfaceof at least one cell wall that is adapted to provide two stable liquidcrystal configurations.
 6. A liquid crystal device as claimed in claim 5wherein the liquid crystal material comprises nematic liquid crystalmaterial.
 7. A liquid crystal device as claimed in claim 5 thatcomprises a surface alignment grating provided on the internal surfaceof the first cell wall and a planar surface treatment provided on theinternal surface of the second cell wall, the arrangement being suchthat one of the stable liquid crystal configurations is a twistednematic configuration.
 8. A liquid crystal device, as claimed in claim 7in which the twist of the liquid crystal material in the twisted nematicconfiguration is greater than 45°.
 9. A liquid crystal device as claimedin claim 7 in which the twist of the liquid crystal material in thetwisted nematic configuration is greater than 90°.
 10. A liquid crystaldevice as claimed in claim 7 wherein the device is latched into thetwisted nematic configuration prior to operation in the second mode. 11.liquid crystal device as claimed in claim 5 that comprises a surfacealignment grating provided on the internal surface of the first cellwall and a homeotropic surface treatment provided on the internalsurface of surface of the second cell wall, the arrangement being suchthat one of the stable liquid crystal configurations is a verticallyaligned nematic configuration.
 12. A liquid crystal device as claimed inclaim 11 wherein the device is latched into the vertically alignednematic configuration prior to operation in the second mode.
 13. Aliquid crystal device as claimed in claim 1 wherein the liquid crystalmaterial comprises a nematic (or long pitch cholesteric) material.
 14. Aliquid crystal device as claimed in claim 13 wherein the liquid crystalmaterial further comprises cholesteric liquid crystal material.
 15. Aliquid crystal device as claimed in claim 14, wherein the liquid crystalmaterial comprises less than 0.02% by weight of cholesteric liquidcrystal material.
 16. A liquid crystal device as claimed in claim 14wherein the ratio of liquid crystal layer thickness to the pitch ofliquid crystal material is within the range of 0.25 to 1.25 inclusive.17. A liquid crystal device as claimed in claim 1 wherein the liquidcrystal material has a positive dielectric anisotropy.
 18. A liquidcrystal device as claimed in claim 1 wherein the liquid crystal materialhas a negative dielectric anisotropy.
 19. A liquid crystal device asclaimed in claim 1 wherein the cell walls comprise electrodes forapplying an electric field to the liquid crystal, the electrodes beingarranged to form a matrix of addressable pixels.
 20. A liquid crystaldevice as claimed in claim 19 that comprises row electrodes formed onthe first cell wall and column electrodes formed on the second cellwall, the first and second cell walls being arranged so as to define anarray of passively addressable pixels.
 21. A liquid crystal device asclaimed in claim 19 that comprises a means of independently applying anelectric field to each individual pixel.
 22. A liquid crystal device asclaimed in claim 21 wherein the means of independently applying anelectric field to each individual pixel comprises a thin film transistorelement.
 23. A liquid crystal device as claimed in claim 22 wherein astorage capacitor is associated with each thin film transistor element.24. A liquid crystal device as claimed in claim 19 which includes atleast one discrete semiconductor circuit capable of addressing two ormore pixels.
 25. A liquid crystal device as claimed in claim 19 whereinthe device is adapted such that the first mode of operation and thesecond mode of operation are capable of simultaneous use on differentpixels or groups of pixels.
 26. A liquid crystal device as claimed inclaim 19 wherein pixels that are to be operated in the second mode areblanked into a predetermined stable liquid crystal configuration priorto operation in the second mode.
 27. A liquid crystal device as claimedin claim 1 wherein the driving voltages applied to the device aresubstantially d. c. balanced over time.
 28. A liquid crystal device asclaimed in claim 1 wherein the second mode is adapted to display imagesat video rate.
 29. A liquid crystal device as claimed in claim 1 whereinthe second mode is adapted to display images with 256 grey-levels.
 30. Aliquid crystal device as claimed in claim 1 comprising at least onepolariser to distinguish between different liquid crystalconfigurations.
 31. A liquid crystal device as claimed in claim 1wherein the device further comprises a reflective means and the deviceis configured to operate in reflective mode.
 32. A liquid crystal deviceas claimed in claim 1 and additionally comprising colour filterelements.
 33. A liquid crystal device as claimed in claim 1 and furthercomprising electronic means for selecting first mode or second modeoperation.
 34. An information display apparatus comprising amicroprocessor unit adapted to electronically address a liquid crystaldevice according to claim
 1. 35. A method of operating a liquid crystaldevice comprising the steps of; taking a liquid crystal device that isarranged such that the liquid crystal material can adopt any one of twoor more stable liquid crystal configurations that will persist in theabsence of an applied electric field, and determining whether to operatethe device in a first mode or a second mode, whereby in the first modeof operation any one of the two or more stable liquid crystalconfigurations can be selected by application of an appropriate latchingvoltage, and in the second mode of operation the application of anelectric field can switch the layer of liquid crystal material from alatched configuration to a switched configuration and in which the layerof liquid crystal material will return to said latched configurationwhen the applied electric field is removed.
 36. A method as claimed inclaim 35 in which the liquid crystal device comprises a plurality ofpixels and in which only some of the pixels are operated in the secondmode.
 37. A method of operating a device as claimed in claim 35 wherebythe liquid crystal material is latched in to a particular stableconfiguration prior to operation in the second mode.
 38. A method ofoperating a zenithal bistable device as claimed in claim 35.